1. Field of the Invention
This invention relates to a DC power source device with a plurality of DC outputs.
2. Description of the Related Art
In a television set, in the case where a mode is shifted from an operation mode to a standby mode, it is required to stop the operation of a television section while preventing the occurrence of a spot leaving. On the other hand, it is required to supply a predetermined DC output to a control microcomputer even when it is in a standby state. A DC power source for meeting these requirements has been proposed in Japanese Patent Unexamined Publication No. Hei. 8-130874.
That is, in this technique, the voltage dividing ratio of a voltage dividing circuit provided in an error detecting circuit is changeable. In a standby mode, control is so made to decrease the voltage dividing ratio. Therefore, if it is assumed that the control target value in the operation mode is for instance 112V, then in the standby mode, the control target value is for instance 40V. Accordingly, when the mode is shifted from the operation mode to the standby mode, the voltage of the DC output (hereinafter referred to as "a first DC output") supplied to the television section is decreased from 112V to 40V. Hence, in the television section, the operation is stopped without the occurrence of a spot leaving. A voltage-drop type regulator is used for a power source for the microcomputer and for instance, the DC output (hereinafter referred to as "a second DC output") which is 27V in the operation mode and 8.5V in the standby mode is decreased to 5V. Accordingly, in the standby mode, even when the voltage of the second DC output is decreased, a standard value of 5V is supplied to the microcomputer.
However, in the case where the above-described power source is applied to a television set with a video cassette recorder, the following problems are involved. That is, the voltage stabilizing control is based on a voltage error of the first DC output which is the power source of the horizontal circuit of the television section. Hence, when the voltage dividing ratio of the voltage dividing circuit is decreased, then the first DC output voltage decreasing control is carried out until the first DC output voltage becomes approximately 40V. Therefore, the power supplied from the primary side becomes approximately zero (0) immediately after the mode is shifted to the standby mode. That is, until the first DC output voltage becomes 40V, the voltage of the second DC output is maintained only by the charge of the smoothing capacitor. As a result, in the case where a voltage-drop type regulator is employed whose input voltage minimum value is 7V for stabilizing the output voltage, the second DC output voltage may become lower than the minimum value of 7V. In the case where the second DC output voltage becomes lower than 7V, the output voltage of the voltage-drop type regulator becomes lower than 5V, and therefore the microcomputer may have troubles in its operation.
Further, in the case of a television set with a video cassette recorder, sometimes the television section is not operated but the video section is operated as in the case of a timer image recording operation or a video cassette ejecting operation. In the case of such an operation, the load current of the first DC output is approximately zero (0) in value. Accordingly, even in the case where the load power of a DC output other than the first DC output is increased, the supply power from the primary side does not show an increase corresponding to the load of the secondary side. As a result, when the motor in the video section is rotated, the voltage of the motor operating power source, namely, the voltage of the DC output is greatly decreased. Therefore, a problem occurs with the rotation of a motor or the like adapted to eject a video cassette.